HLB2 gave you a tongue in cheek response, but I'll answer you very directly. To make sure we're on the same page, I'm going to define it first and then highlight the typical situation.

By definition a OPP (one penis policy) is a non-monogamous rule where a particular relationship only allows for the non-monogamy to be practiced with female partners.

Your typical OPP is usually found in MF couples where the M pushes for a OPP as a condition of being non-monogamous, meaning that the F can only date other women.

Why does it bother someone? How is it homophobic?

I'll go step by step through this...

1. It treats potential ENM partners differently based on their gender. So we must ask...why?
2. The answer is because they're viewed differently, women are viewed as less threatening.
3. They're viewed as less threatening because same sex relationships are not seen with the same legitimacy as hetero relationships.
4. If same sex relationships are not seen as equally legitimate, that's homophobia.

It bothers people because of two key things:

• It's homophobic.
• It's a sign of insecurity, indicating that the person hasn't done the work to be non-monogamous.
• Bonus: It's also often hypocrisy, because frequently the same man that will require a OPP will often want to engage in hetero ENM partnerships.
  

Just to address the most common responses:

• "Well she's bi, so we're just ENM for her to explore that" - I get it, this makes sense on the surface...until you realize that it still assumes that the women she dates are not threatening to your relationship, but that dating/playing with men would be.
• "It's not homophobia, it's just insecurity, he doesn't want to have to compete with another guy" - Yeah, it's definitely insecurity. But it's also homophobia, because otherwise he'd be threatened by the women too.